1. Field of the Invention
The present invention relates to a method and apparatus of dressing a grinding wheel, and more specifically to a method of effectively dressing a high grain concentration grinding wheel which is usable for ultra high precision processing.
2. Description of the Prior Art
As shown in FIG. 7, when a grinding wheel 1 is formed into a predetermined shape (referred to as truing or trued, hereinafter), since abrasive grains 2 contained in the grinding wheel 1 are ground down and thereby buried under a surface 3 of the grinding wheel, it is necessary to form new cutting blades by dressing the surface 3 of the trued grinding wheel 1 again, so that binding material 4 can be cut away and thereby the abrasive grains 2 can be exposed from the surface 3 of the grinding wheel 1, as shown in FIG. 8.
Here, in order to form new cutting blades by cutting off the binding material 4 without deforming the trued grinding wheel 1, it is indispensable to set the hardness of the abrasive grains contained in a dressing tool to a value lower than that of the abrasive grains 2 contained in the trued grinding wheel 1 but higher than that of the binding material 4 of the same trued grinding wheel 1.
Further, it is also necessary to set the grain diameter of the abrasive grains contained in the dressing tool to such an appropriate grain diameter that although the abrasive grains contained in the dressing tool can enter into an appropriate depth between the abrasive grains 2 contained in the trued grinding wheel 1, the abrasive grains 2 may not fall away from the surface thereof, in relation to an averaged interval between the abrasive grains contained in the trued grinding wheel 1.
Here, when the dressing tool which contains the abrasive grains having appropriate hardness and a grain diameter is brought into frictional contact against the surface 3 of the trued grinding wheel 1 to be dressed, the cutting blades can be newly formed without deforming the trued shape of the grinding wheel 1, so that it is possible to manufacture an abradable grinding wheel 1 effectively.
However, in the case of the grinding wheel used for ultra high precision processing, the grain diameter can be smaller than several microns because of a high wheel number. Further, the grain concentration is high and thereby the averaged grain interval is extremely small. Therefore, the grain diameter contained in the dressing tool used to dress the grinding wheel for ultra high precision processing must be extremely small.
On the other hand, since the dressing tool is usually sintered without use of any binding material, there exists such a tendency that the grains are diffusion-coupled with each other and thereby the grain diameter increases. This tendency increases as the grain diameter is decreasing. As a result, it has been difficult to manufacture a dressing tool used to dress the grinding wheel containing ultra-fine grains which is usable for the ultra high precision processing.
To overcome this problem, conventionally, the grinding wheel for the ultra high precision processing is trued so far often for dressing purposes. However the dressing efficiency is not high and the wheel surface is easily made flat, this problem so far arises in the ultra high precision processing.
On the other hand, a method of dressing the grinding wheel by electrolytic processing has been developed. However, there exist the other problems in that the power unit for the electrolytic processing is relatively costly and further it takes a relatively long time to find out the optimum conditions of the electrolytic processing.